Originally Posted by crocus
If you use one of the drive-on jacking aids, that puts all of the weight on one axle. We only have 4000# rubber axles, so that would overload the one by a factor of 50%.
While on the surface that might sound logical, it not really true.
With 4 wheels on the ground the CG is calculated by drawing lines that connect each tire (forming a rectangle). The CG is in the exact center of the rectangle. This can be easily found by drawing an X through the 4 wheels. Since the intersection is equidistant from each wheel, each wheel is carrying an equal amount of weight. (ie 1/4th of the total load)
With 3 on the ground the actual load on each wheel gets a bit more complicated. We need to find out the new CG location to determine the actual load on the wheels in contact with the ground. To do this lines are drawn connecting the wheels as before, but it now forms a right triangle with an 8 foot side and a side that connects the centers of the tandem.
Use geometry to determine the center of the triangle (draw lines bisecting the angles of the triangle and the CG is the intersecting point). The actual load on each wheel is then calculated using vectors to each wheel. If the 3 wheels are equidistant apart (8 foot x 8 foot say), the weight on each wheel would be exactly 1/3 of the total camper weight. Since they are not, the load of the missing wheel is shared unequally by the 3 remaining tires. (slightly more on the side with 2 tires - shared by the 2 tires equally; slightly less on the side with one tire)
While the tires are still "overloaded" per se, that load rating is calculated to be the maximum load at 60 MPH and 50 PSI (for load range C); not a static load. This is well within the capability of the tire for brief periods.
That is why you can drive on a blowout to a safe spot without immediately blowing all the others (just not at 75 MPH!). The slower as safely possible the better.
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